The novel ternary phase CaAl2±xCr2±xO7 in the Al2O3–CaO–Cr2O3 system: What is the chromium solubility?

Recently, in the Al 2 O 3 –CaO–Cr 2 O 3 system, a Cr 3+ ternary phase named wuboraite (CaAl 2 Cr 2 O 7 ) was synthesized for the first time in an air atmosphere, which later showed an undetermined hint of solid solubility between Cr 3+ and Al 3+ in different atmosphere. In the present investigation, the solubility limits (or x values) of the CaAl 2± x Cr 2± x O 7 were determined by conducting high-temperature equilibrium experiments in both air and N 2 atmospheres with the various proportionate mixtures of CaO, Al 2 O 3 , and Cr 2 O 3 , and subsequently using powder X-ray diffraction (XRD) and scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS) analysis. In the air atmosphere, the Cr richest end-member of the Wuboraite solid solution phase is CaAl 1.77 Cr 2.23 O 7 (or x  = 0.23), while that of Al richest end-member is CaAl 2.18 Cr 1.82 O 7 (or x  = 0.18). Whereas, in the N 2 atmosphere, the Cr richest end-member of the wuboraite solid solution phase is CaAl 1.77 Cr 2.23 O 7 (or x  = 0.23), while that of Al richest end-member is CaAl 2.15 Cr 1.85 O 7 (or x  = 0.15).